In his letter, O'Brien recalls first meeting Cornell in 1993 before mixing Soundgarden's seminal Superunknown album (1994). While Cornell was "a bit intimidating" at first, the two quickly struck up a friendship that would last more than two decades.

"He was a great singer, tall, brooding and a bit aloof who was a fairly no-nonsense person that didn't suffer fools particularly well," O'Brien wrote. "A pro. A smart-ass. My kinda guy."

O'Brien recalled numerous encounters with Cornell over the years, producing Audioslave's final album and Cornell's 2015 solo album A Higher Truth.

"He was a titan among singers who could summon his inner self and demons in a way that was powerful, ferocious and believable all the while making it look easy," O'Brien continued. "Yet, he was also able to sing barely above a whisper with just an acoustic guitar (by the way, a very underrated guitarist) and give you the same emotion, power and vulnerability. Again, making it look easy."